A DC-DC converter is an electronic circuit that converts a source of direct current (DC) from one voltage to another. For example DC-DC converters are widely used in portable devices to provide power from a battery. DC-DC converters may also regulate the output voltage, compensating for varying load current and variations in the input voltage.
FIG. 1A illustrates an example embodiment of one common type of DC-DC converter. The DC-DC converter circuit 100 in FIG. 1A (simplified to facilitate illustration and description) is a switching step-down converter (the Input voltage is higher than the output voltage), and the basic design is called a Buck converter. A power source 102 provides direct current at an input voltage VIN. The circuit 100 provides direct current to a load (R) at an output voltage VOUT. There are two electronic switches (SW1, SW2), only one of which is activated (closed) at any one time. SW2 may be a diode instead of a switch, and the diode is “activated” when positively biased. When SW1 is closed, current flows into R and a filter capacitor (C) from the source 102, and VOUT rises linearly. In addition, when SW1 is closed, energy is stored in L and C. When SW2 is closed, current flows from stored energy in C and from stored energy in L, and VOUT decreases linearly.
FIG. 1B illustrates an example embodiment 104 of a DC-DC boost converter circuit (the input voltage is lower than the output voltage). The primary difference between the circuit 100 of FIG. 1A and the circuit 104 of FIG. 1B is the location of the switches (SW1, SW2) relative to the inductor L. When SW1 in circuit 104 is closed, energy is stored in L and load current is provided by C. When SW2 in circuit 104 is closed, load current flows from VIN and from stored energy in L, and energy is stored in C. In Circuit 104 of FIG. 1B, SW2 may be a diode instead of a switch.
FIG. 1C illustrates an example embodiment 106 of the DC-DC converter circuit 100 of FIG. 1A with the addition of feedback to control the output voltage. An amplifier 108 amplifies the difference between VOUT and a reference voltage VREF. A ramp generator 110 receives a clock signal (CLK) and generates a constant-frequency ramp signal. A comparator 112 compares the output of the amplifier 108 to the ramp signal. A driver circuit 114 activates at least SW1 (SW2 may optionally be a diode). The width of the pulse driving SW1 (called “on-time”) is determined by the time at which the output of the amplifier 108 is equal to the rising ramp voltage. During the remainder of the clock cycle (“off-time”) the driver 114 activates switch SW2 (or a diode conducts while forward biased).
There are many variations in topology and control of DC-DC converters. Some converters have multiple inductors in a resonant circuit. The circuit illustrated in FIG. 1C uses output voltage feedback. Some converters use current feedback, or other feedback signals such as a ripple voltage. Some converters have multiple feedback loops, in general, there are advantages and disadvantages of each variation, and some systems have special requirements, in particular, power supplies for advanced digital circuits have a challenging set of requirements. Microprocessor cores, digital signal processors, and other devices may switch rapidly from sleep-mode to full-power and full-power back to sleep-mode, requiring a fast response by the power supply to sudden changes in load current.
In general, there is a need to extend the high frequency response of a DC-DC converter while maintaining stability. This is especially true when significant loads may be switched in and out of standby mode to reduce power, in the embodiment of FIG. 1C, the amplifier 108 commonly includes a compensation filter (not illustrated), where “compensation” means that the feedback network gain and phase as a function of frequency ensure that the overall system with feedback is stable. However, for a system as depicted in FIG. 1G with constant-frequency switching, the system cannot respond to a sudden load change until the next clock cycle. An alternative way to handle rapidly changing current demands is to maintain a constant on-time (or off-time), and instantaneously change the switching period. Allowing the switching period to instantaneously change enables a faster response.
FIG. 1D illustrates an alternative embodiment having a constant on-time with variable off-time and variable switching frequency. In FIG. 1D, as in FIG. 1C, the difference between the output voltage VOUT and a reference voltage VREF is amplified by amplifier 108 to generate a voltage error feedback signal. In FIG. 1D, the voltage error signal drives a voltage controlled oscillator (VCO) 116. The output of the VCO 116 is a series of poises at the switching frequency. At the beginning of each cycle of the output of the VCO 118, a pulse generator (“one-shot”) 118 generates a constant-width on-time pulse. The driver circuitry 120 drives SW1 during the on-time pulse, and then drives SW2 during the remaining portion of each cycle from the VCO 116.
The example circuit of FIG. 1D allows the switching period to instantaneously change, thereby enabling a faster response to a load change. A further advantage of the circuit of figure ID is that it does not need a compensation filter. However, the average switching frequency of the DC-DC converter in the circuit of FIG. 1D varies widely with load current and other power system parameters, and there are multiple system requirements that need a constant switching frequency, or at least a quasi-constant switching frequency. For example, L, C, and other components needed to filter the output voltage ripple need to be optimized for a particular ripple frequency (or a least a relatively narrow range of frequencies). In addition, suppression of system radio frequency interference (RFI) may require external system components that are optimized for a particular switching frequency (or at least a narrow range of frequencies). In addition, the efficiency of the DC-DC converter may be optimized at a particular switching frequency (or at least a narrow range of frequencies).
There is an ongoing need for a DC-DC converter with a rapid response to transient load conditions, and a narrow range of switching frequencies.